session: Don't assert non-existance of incomplete session
session: Don't assert non-existance of incomplete session

If a session doesn't have an ID, there is no way it could have been
stored in the session table, and checking with a NULL key results in a
NULL pointer dereference.

Closes https://github.com/flatpak/xdg-desktop-portal/issues/186

matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal